1. Field of the Invention
This invention relates generally to a tone arm assembly, and is directed more particularly to a novel tone arm assembly which is controlled with an electromagnetic force.
2. Description of the Prior Art
There is already known such a tone arm that an electro-magnetic force, for example, a driving force of a motor is utilized to move a tone arm in the horizontal and vertical directions relative to the surface of a record disc. The tone arm assembly of this kind requires a so-called positioning servo circuit in order to stop the tone arm at a predetermined position on the surface of a record disc and to start the reproducing. However, a system of using the positioning servo only has a drawback in that when the tone arm is rendered to stop on the surface of the record disc at a desired position, a vibration is apt to occur. In other words, a mere positioning servo provides a strength of stability in proportion to a distance from the desired stop position. If a position of a tone arm is taken as x.sub.i, an initial velocity as v.sub.i, and a desired position as a reference, the equation of motion can be expressed as follows: ##EQU1## A solution of the above equation is obtained as follows: ##EQU2## where .omega.=.sqroot.k/m.
As apparent from the above solution, in case of using only the positioning servo, there is no attenuation term in the above equation so that the tone arm becomes vibratile and will not stop at the desired position. Therefore, it is considered to provide a mechanical resistance to its rotating shaft in order to stop the tone arm at the desired position. However, the mechanical resistance is irregular at every mechanical portions and also a frictional force is exerted on the tone arm during its movement to cause a mechanical loss. Further, the assignee of this applicant has already proposed a method in U.S. Ser. No. 923,455 such that a motor is first driven at a constant velocity with a brake current being supplied thereto to move a tone arm from its rest position, and when the arm reaches a predetermined position, an arm driving circuit is cut off to stop the arm at the predetermined position by using a position detecting pulse generator, a counter for counting the pulse, and a D-A converter. According to this method, the arm can be stopped more smoothly as compared with the case of providing a mechanical resistance or frictional force. However, it requires the digital circuit for counting pulses, the converter for converting a digital signal into an analog signal, and a tank circuit for preventing ringing. Further, a positioning servo circuit is necessary in addition to a constant velocity servo circuit for the arm. As a result, its circuit arrangement becomes complicated.